Neutron diffraction investigation of DyFe4Al8 and HoFe4Al8

Journal of the Less-Common Metals, 94 (1983) 205-212

NEUTRON HoFe,Al,*

W. SCHAFER Mineralogisches

DIFFRACTION

INVESTIGATION

205

OF DyFe,Al,

AND

and G. WILL Institut der Universitiit Bonn, Poppetsdorfer

S&loss, D-5300 Bonn I (F.R.G.)

(Received March 21,1983)

Summary

Neutron diffraction experiments were performed on polycrystalline samples of the ternary intermetallic rare earth compounds DyFe,Al, and HoFe,Al, in the temperature range I.6300 K. The compounds crystallize in the tetragonal space group I4/mmm (0::). The structural parameters were refined from the nuclear scattering intensities. The magnetic ordering behaviour was evaluated from low temperature magnetic scattering data. For both compounds we found only one magnetic phase transition at about T = 25 K and attributed it to an ordering of the iron sublattice in a conical spiral structure. The propagation vector of the spiral is along [llO], and the rotation angle of the iron moments is about 45” for DyFe,Al, and 35” for HoFe,Al,. The magnitude of the ordered iron moment is 3.6 ur,per iron atom at 4.2 K. The neutron diffraction data reveal no indication of a magnetic ordering of the rare earth sublattice. The neutron diffraction results are discussed in the context of contradictory interpretations of magnetization measurements.

1. Introduction

The magnetic properties of the ternary compounds RFe,Al, (R ss rare earth) have been investigated using magnetic susceptibility and Mossbauer spectroscopy experiments by Buschow and van der Kraan [l] and Felner and Nowik [Z]. From these measurements it was concluded that these compounds generally exhibit two independent phase transitions: an antiferromagnetic ordering of the iron ions (TN > 100 K) followed by a magnetic ordering of the R sublattice at lower temperatures (!I’, c 50 K). We were able to verify this description in an earlier neutron diffraction study of compounds with R c Er and Tb [a]. In the present paper we wish to report the neutron diffraction results for DyFe,Al, and HoFe,Al, which contradict the previous interpretation. * Paper presented at the Sixteenth Rare Earth Research Conference, The Florida State University, Tallahassee, FL, U.S.A., April 18-21,1983. 0 Elsevier Sequoia/Printed

in The Netherlands

206

2. Experimental details The neutron diffraction measurements were performed on polycrystalline material prepared by argon arc melting and vacuum annealing. The compounds were kindly provided by K. H. J. Buschow, Philips Research Laboratory, Eindhoven. Neutron diffraction patterns were recorded on cylindrical samples of diameter 7 mm and length 30 mm using the Katinka di~actometer at the Kernfors~hungsanlage Jiilich. The wavelength used was 1.206A. Diffractograms were recorded at temperatures of 300,77 and 4.2 K (Fig. 1); for DyFe,Als an additional diffractogram was recorded at 1.8K. The temperature dependence of the magnetic ordering process was investigated using a variable-temperature cryostat.

~~RTTERING

RNGLE

2 THETR

IN

DEGREES

Fig. 1. Neutron scattering diagrams of HoFe,Al, (a) at 300 K and (b) 4.2 K. Satellite splitting of the (200,101) reflection is indicated by the difference intensity shown in the inset.

3. Crystal structure data The isostructural RFe,Al, compounds crystallize in a body-centred tetragonal cell (space group, 14/mmm @I:,‘)) with two formula units per unit cell.

207

The atoms occupy the following positions: R in 2a (0, 0,O);iron in 8f (& $,$); Al(l) in 8i (x,0,0); Al(2) in 8j (x,$0). The structure is depicted in Fig. 2. The lattice parameters were refined by least-squares analysis of the nuclear peak positions in the neutron diffraction patterns. The results are listed in Table 1.

Fig. 2. Crystallographic unit cell of the RFe,Al, compounds: iron atoms; small open circles, aluminium atoms. TABLE

large circles, R atoms; shaded circles,

1

Crystal structure parameters refined from the nuclear neutron data DyFedAla

HoFedAla

a (A) c (A) V(A3)

-4W)) xW(2)) B K

T=3OOK

T=

8.749 f 0.005 5.049 f 0.003 386.5

8.729 kO.004 5.042 kO.004 384.2

0.336 f 0.003 0.282 k 0.003 0.4kO.2 6.49kO.18

4.2K

T=3OOK

T= 4.2K

8.767f0.001 5.047 f 0.003 387.9

8.743kO.003 5.032 + 0.003 384.6

0.340 f 0.004 0.275 ~0.004 0.5 * 0.2

1.29+0.04

The room temperature intensities were used to refine the atomic parameters and to determine the scaling constant K and the temperature factor B. The least-squares analysis was performed using our program POWLS [4] with the integrated observed intensities obtained by gaussian profile analysis of the neutron peaks. The refinement calculations performed using 14 observations containing 30 reflections resulted in R values of 4.7% for DyFedAl, and 3.0% for HoFe,Al,. A comparative list of observed and calculated intensities is given in Table 2. The refined parameter values are included in Table 1.

4. Magnetic structure analysis The magnetic ordering behaviour is evaluated using low temperature neutron diffraction experiments. We detected no additional magnetic scattering in the 77 K diagrams of either DyFe,Al, or HoFe,Al,. Thus we find no indication

208

TABLE 2 Comparison of observed and calculated and HoFe,Al,

hkl

110

intensities for the crystal structure refinement of DyFe,Al,

HoFedAls

DyFelAld I(obs)

&ale)

Ifobs)

I(calc)

1437

1546

541

949

1485

1426

2013

1901

6562

6784

200 101 220 211 310 301 002 112

(Al obscured)

490 321 202

20684

330

(Al obscured)

436

21192

713

(Al obscured)

75194

(Al obscured)

271

74630

1274

420 411 222

5033

5318

43759

42618

312

1389

1265

2382

1727

510

632

490

759

701

501 431 402 103

1654

1346

3426

3600

332

861

849

1804

1529

440 521 422 213

9264

9246

27720

26664

530

695

693

1032

1314

600 303

1011

881

2739

2659

of a magnetic phase transition above 77 K. We observed a moderate enhancement of the intensity of a number of reflections with h, k, 2 = 2n and the simultaneous appearance of some satellite peaks indicating a spiral spin confi~ation in the 4.2 K patterns. The most pronounced satellite splitting was found for the (200) and (220) reflections which are plotted in Fig. 3 for HoFe,Al,.

4’

; x * 9

(2281 L2111

T14.2K

! ,

/ I “1

\

1

%2 L

<22e1-

; /f

: 1

\, _.

. . . . ..-.~-_ 22 Sc.tt*r,np

21

._-z 2.4

23 (28,

R”Ql.

(b)

Fig. 3. Comparison of (a) the (200,101) reflection and(b) the (220,211) reflection at 300 and4.2 K. The satellite splitting of the 4.2 K intensities was calculated by gaussian profile analysis. TABLE 3 Calculated magnetic scattering for fundamental spin configurations hkl

(sin 6)/J.

Ferromagnetic

100

0.057 0.081 0.099

100 -

200 101 111

0.114 0.114 0.128

47 24

210 201

0.128 0.151

220 211

0.162 0.162

300

0.172

310 221 301

0.181 0.190 0.198

002

0.198

-

-

60

-

Antiferromagnetic

Antiferromagnetic

loo

100

-

-

-

-

-

LOO -

100 -

100 -

-

-

39 -

-

-

-

31

0

29 100 -

-

13 69

-

19

-

-

38 Q _

18

10

19

19

-

0

8

-

-

9

-

100

57 35 34

14 36 14

32

Ferromagnetic

-

-

21 52

unit cell

Fe s~bl~ttice

Ho s~b~att~e

110 001

in the RFe,Al,

-

Each model is scaled in~vidually. Each spin confi~ration (0”) and perpendicular (90”) to the tetragonal c axis.

_

_

_

-

_ -

_ 0

is calculated

21

-

_

25

_ -

25

-

for spin directions parallel

We started the analysis of the magnetic scattering by calculating the magnetic reflections for some fund~ental collinear spin configurations based on the crystallographic unit cell: a ferromagnetic or antiferromagnetic order-

210

ing of the two R atoms and a ferromagnetic or antiferromagnetic ordering of the eight iron atoms (the antiferromagnetic spin sequence assumed for the iron atoms is that found earlier in ErFe,Al, and TbFe,Al, [3]). The individually scaled model intensities are listed in Table 3. By comparing the model calculations with the experimentally observed magnetic scattering we established that only the iron sublattice is involved in the magnetic ordering. The observed enhancement of reflections with even h, k and 1 corresponds to a ferromagnetic component of the iron sublattice. The additional satellite splitting of these reflections is in accordance with a spiral component of the iron moments with a propagation vector along the [llO] direction. The corresponding satellite splitting is depicted in Fig. 4 as a function of the propagation vector length (rotation angle of the iron moments) for the (200) and (220) reflections. The observed splitting corresponds to a moment rotation of about 45” for DyFe,Al, and of about 35” for HoFe,Al,. The observed magnetic intensities are used for the determination of the magnitude of the iron moments. In a conical spiral structure with half-cone angle cp the magnetic scattering length p is defined by p = pfcosrp for the fundamental peaks and p = pf sin cp for the spiral component, where p is the magnitude of the ordered moment and f is the magnetic form factor. A quantitative analysis of the magnetic intensities performed for the HoFe,Al, data revealed a magnetic moment of 3.6 f 0.3 ug per iron atom at 4.2 K. The half-cone angle of the spiral is determined to be c1= 45”. The angles of the cone axis with the scattering vectors [200] and [220] are both 45” ) 5” and that with the scattering vector [002] is 60” f 20”. Thus the cone axis corresponds to the [111] direction of the crystallographic cell. A comparison of observed and calculated intensities is given in Table 4.

5. Discussion The neutron diffraction results for the magnetic ordering behaviour in DyFe,Al, and HoFe,Al, presented here contradict the previous interpretations of susceptibility and Mossbauer measurements [l, 2,5]. In these interpretations an antiferromagnetic ordering of the iron atoms was concluded to occur at T N x 130 K. A second transition at T x 25 K was attributed to a magnetic ordering of the R sublattice into a ferromagnetic state [l, 53 or into an antiferromagnetic,state [a]. Our neutron diffraction measurements reveal only one magnetic transition at T w 25 f 5 K, which is attributed to a conical spiral structure of the iron atoms. The ordering temperature was measured by following the temperature dependence of the satellite reflections (Fig. 5). There is no indication of an additional ordering ofthe rare earth atoms at 4.2 K (down to 1.8 K for DyFe,Al,). Special emphasis was placed on the (110) and (100) reflections, which are very sensitive for a ferromagnetic or an antiferromagnetic ordering respectively of the R sublattice (see Table 3), but no magnetic contribution was detected. One possible explanation for the different magnetic ordering behaviour is

211

TABLE 4 Comparison hkl

(200) (2OO)O (200) + (220) (22O)O (220) + (002)O*

of observed and calculated

magnetic intensities for HoFe,Al,

26(obs)

20(calc)

(deg)

(deg)

14.9 15.8 16.6 21.2 22.4 23.6 27.5

15.0 15.8 16.6 21.3 22.4 23.6 27.6

(2’ = 4.2 K)

Z(obs)

Z(calc)

5000 7300 5800 3200 3400 3300 1000

5898 7761 5821 2536 3182 2308 891

,

-

1

t

I% “I

J.

,::I

‘Ittt’t’lt f’l.*’ t [ttttt, ‘t/t ft $,t f

ttttlftt,tt,

(‘ii

Fig. 4. Calculated satellite splitting of the (200) and (220) reflections as a function of the rotation angle of the iron moments with a propagation vector in the [llO] direction (,I = 1.206 A). Fig. 5. Temperature dependence of the (200)+ and (220)+ satellite intensities in HoFe,Al, an ordering temperature 2’ = 25 + 5 K.

indicating

the existence of crystallographic disorder, which is quite probable in ternary intermetallic compounds of this type. The nuclear scattering of neutrons is very sensitive for the analysis of structural disorder in view of the various scattering lengths of the atoms involved: b(Dy) = 17.1 fm, b(Ho) = 8.5 fm, b(Fe) = 9.54 fm and b(A1) = 3.449 fm. On the basis of the analysis of the nuclear intensities (see Section 3) we can exclude any substantial crystallographic disorder for the samples provided for our neutron diffraction experiments. Despite the contradictory interpretations of the magnetization and the neutron diffraction experiments, some common features should be emphasized. All methods reveal a magnetic phase transition at T = 25 K. The value of 3.6 uB per iron atom calculated for the magnetic moment from our neutron diffraction

212

data is in very good agreement with an effective moment of 3.8 u, per iron atom obtained from the magnetization measurements in the paramagnetic region reported by Buschow and van der Kraan [l]. There is further agreement concerning the magnetic structure of the iron sublattice. Felner and Nowik [2] conclude that an oscillatory structure rather than a simple antiferromagnetic moment orientation may be present. This statement agrees with our observation of an incommensurate structure of the iron sublattice. A critical review of all observations, including thermomagnetic history effects during the magnetization measurements, would presumably reveal a similar interpretation to that of the neutron diffraction analysis.

Acknowledgments We should like to thank F. Elf and E. Jansen for assistance with the data analysis. Financial support by the Bundesministerium fur Forschung und Technologie, Bonn, for the neutron diffraction experiments is gratefully acknowledged.

References 1

2 3 4 5

K. H. J. Buschow and A. M. van der Kraan, J. Phys. F, 8 (1978) 921. I. Felner and I. Nowik, J. Phys. Chem. Solids, 39(1978) 951. M. 0. Bargouth, G. Will and K. H. J. Buschow, J. Magn. Magn. Mater., 6(1977) 129. G. Will, J. Appl. Crystallogr., lZ(1979) 483. G. Will, E. Jansen and W. Schiifer, Rep. Jill-1646, 1980 (Kernforschungsanlage Jiilich). P. C. M. Gubbens, A. M. van der Kraan and K. H. J. Buschow, J. Magn. Magn. Mater., 27 (1982) 61.